Case-hardened, worked steels



cann -peak 2,881,109 CASE-HARDENED, WORKED STEELS No Drawing. Application October 22, 1956 Serial No. 617,272

13 Claims. c1. 14s -12.1

This invention relates to the improvement in me-' chanical and physical properties of steel by a process capable of being employed in the cold-finishing of steel, as in the cold-finishing of steel bars, rods, wires, and the like steel products.

It is an object of this invention to produce and to provide a method for producing steel products having new and improved physical and mechanical properties, with new and different surface chemistry characteristics and properties.

More specifically, it is an object of this invention to provide a method applicable to the processing of steels of the type which strain harden and harden by some mode of precipitation when worked at elevated temperature to improved and to change physical and mechanical properties of the steel, and it is a related object to produce a new and different steel product having new and improved combinations of physical and mechanical properties, such as resistance to wear, corrosion, fatigue, etc.

This application is related to the copending applications Serial Nos. 518,411, 518,412, 518,413, and 518,414,

nited States Patent C filed June 27, 1955 now Patents No. 2,767,837, No.

2,767,835, No. 2,767,836, and No. 2,767,838, respectively.

The inventions of the aforementioned copending applications are addressed to the method for the improvement of physical and mechanical properties of steel by the process which includes the step of working the steel as by advancement of the steel through a die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, or at a temperature within the range of 200 F. to ll00-1200 F. In accordance with the teaching of the aforementioned copending applications, the temperature of the steel in the elevated temperature reduction step has considerable influence on the improvements secured in physical and mechanical properties of the steel. By .controlling temperature, chemistry, and amount of reduction, itis possible selectively to adjust physical and mechanical properties developed in the steel over a fairly wide range while enhancing the uniformity of physical and mechanical properties in the steel from heat to heat by comparison with hot-rolled steels.

This invention embodies the concepts set forth in the aforementioned copending applications in combination with the treatment of the steel prior to taking the elevated temperature reduction step to effect a diflusion transfer at elevated temperature followed by quenching or otherwise cooling the steel from diffusion temperature to ambient temperature, or to approximately the temperature for advancing the steel directly to the die for the elevated temperature reduction step. The term diffusion transfer will hereinafter be defined with reference to the previous treatment of the steel to case the steel, as by means of carburizing, but it will be understood that other diffusion transfer processes may instead be em- 2,881,109 Patented "Apr; 7, 195g:

. 2 ployed, such as nitriding, cyaniding, carbonitriding, and the like, to introduce elements which change the chemistry of the steel, at least in the surface portions.

As used herein, the term physical and mechanical properties" is meant to include tensile strength, yield strength, hardness, ductility, elongation, residual stress, warpage, corrosion resistance, wear resistance, and the like. It has been found, for example, as described in the aforementioned copending applications, that machine ability characteristics of the steel and such properties as tensile strength,--yield strength, proportional limit, imv pact strength, and hardness are more beneficially affected when, as described in the copending applicationsserial Nos. 518,414 and 518,413, the steel is advanced through the die to elfe'ct reduction in cross-sectional area whilethe steel is at a temperature within the range of 450-8501 F. These same concepts carry over into the process described and claimed herein wherein the elevated tem" perature reduction step is employed in combination with the previous step of carburizing the steel or providing another case by diffusion transfer processes well known to the art.

As in the aforementioned copending applications, the trend of the improvements in physical and mechanical properties, and the extent of improvements in any one of the properties, is influenced by the temperature of the steel being reduced, the chemistry of the steel, and the amount of reduction that is taken. The rate of cooling of 'the steel after elevated temperature reduction has very litle influence on the properties and character of the steel with the exception that lower stress levels are secured and a preponderance of compressive stresses can be provided in the steels when the steels are rapidly cooled, as by quenching following the elevated temperature reduction step, and especially when the elevated temperature reduction step is taken while the steel is at a temperature within the range of 750 F. to the lower critical temperature of the steel composition.

Steels that respond in the manner described to the com bination of steps of diffusion transfer, as by carburizing, prior to elevated temperature reduction can be classified quite generally as steels of the type which strain harden and harden by some mode of precipitation or other rearrangement when worked at a temperature within a range of 200 F. to the lower critical temperature for the steel composition. Typical of the class of steels are the hot rolled, non-austenitic steels which have a pearlitic structure in a matrix of free ferrite.

For effecting reduction at elevated temperature (ETD),

., the steel may be advanced through a die such as adraw die, extrusion die, or roller die to elfect reduction in cross-sectional area. While the process of rolling is not equivalent to the process of drawing or extrusion, it has been found that many of the characteristics and properties capable of being developed in steels in the elevated temperature reduction step are also capable of development when the elevated temperature reduction step is taken in a rolling operation while the steel isat a temperature within a range of 200 F. to the lower critical temperature of the steel composition.

The process cycle of carburizing followed by elevated temperature reduction provides a unique diffusion reduction concept which produces steels having a hard, high 3 capable of continuous, rapid,, and economical operation *9. R S P S e ls h n .imnmys ...prcpsrtie tas -9 9- pared to conventional cold drawn carburized steels, or elevated temperature drawn carburized steels.

we h r n t e ter ,e fius qi and c r zi are intended to have their normal meaning in the steel trade. Difiusion ean be defined briefly as involving the movement of atoms within a solution. The net movement is usually in the direction from regions of high concentration toward regions of lower concentration in the attempt to achieve homogeneity of the system, which may be a liquid or gas or, as in the present instance, a solid metal. Carburizing is a diffusion-transfer process wherein carbon is introduced into the steelby bringing th'e'steelintointimate contacting relation with a car- 'ac" 'us material,'i11 the form of a solid, liquid, or gas, an heatrn'gto a temperature above thetransformation ti for "the steel holding at that temperature for anhmountl'of tirne'suflicient to "give the desired case to the steel. Qarburi'zinfg is'generally followedbyquenching to produce a hardened case. The time and temperature relationship influences'the depth of case, as is well known inthe art.

Carburizing eliminates the decarb in the steel and hardens the surface of the steel, thereby markedly 'to increase the endurance limit of the elevated-temperaturedrawn steel by comparison with the results secured with steels having decarb still in the surface. Carburizing increases the amount of carbon in the surface of the steel. The case may be formed to various depths, depending on the time and temperature of carburization. Ordinarily the case will be formed to depths of about 0.030" to 0.060", but the desired results may be secured by elevated-temperature reduction of steels having a depth of case within the range of 0.001" to 0.100".

The treatment after carburizing can be varied. The steel can be quenched rapidly to cool the steel to room temperature, followed by reheating the quenched steel to the elevated temperature desired for taking the elevated temperature reduction step. The carburized steel can be air cooled to ambient temperature and then reheated to the temperature desired for the elevated temperature reduction step. The carburized steel can be quenched or air cooled to temperatures above ambient temperature, such as to 'a temperature slightly below the temperature desired for elevated temperature reduction, followed by reheating to the desired temperature, or the carburized steel'can be quenched or otherwise cooled to temperatures corresponding to thetemperature desired for the elevated temperature reduction step, followed immediately by reduction of the steel at the desired temperature. While elevated temperature reduction of the carburized and quenched steel is intended to include reduction of. the steel while at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, it 'is preferred to effect the desired reduction in cross-sectional area while the steel is at a temperature within the range of 450 F. to the lower critical temperature for the steel composition.

The concepts of'this invention will hereinafter be illustr'a'ted by reference to the processing of two steels which can be taken as representative. These includeC-1144 and 4140 having the following ladle analyses in which the major ingredients, other than iron, are set forth:

The procedures for processing the'st'eels for illustrating the practice of this invention will now briefly be described.

'H'ot "rolled steel bars as received were descaled pickling in sulphuric acid and limed to prevent rusting. The hot rolled, pickled, and limed bar stock was packcarburized (Houghton Quicklite A carburizing compound) and heated to a temperature of about 1600 F. for twelve hours. The carburized material utilized for the stepquenched process followed by immediate drawing was quenched in a salt bath to various temperatures for extraction of heat from the carburized steel. Another group of the carburized material was air cooled to room temperature and then reheated in a suitable heat treating furnace to the desired temperature for taking the elevated temperature reduction step.

The steel bars to be drawn were lubricated in advance of drawing, and then the steels were advanced through a draw die to take a reduction in cross-sectional area. For use in the practice of this invention, the bars of 0-1144 steel were ,4 in diameter, and the bars of the 4140 steel were originally ,5 in diameter. V

The properties for which data has been developed are intended to have their usual meaning in the steel art. As used herein, the term warpage factor is directly related to residual stresses. The warpage value is an indication of the concentration and character of the longitudinal stresses present in steel. The residual stress is obtained by means of warpage test wherein the length of the test piece is determined as being five times the diameter plus two inches. The test pieces are slotted through a diameter for a distance five times the diameter. The length of the slot is recorded and the maximum diameter perpendicular to the slot is also recorded. The differences between the diameter before slotting and after slotting represent the flare caused by the presence of residual stresses. The flare is considered positive, indicating a preponderance of tensile stresses at the surface of the steel, if the bar expands upon slotting. The flare is considered negative, indicating a preponderance of compressive stresses at the surface of the steel, if the ends move toward the cut made through the diameter. The warpage values determined are calculated on the following equation:

Warpage factor: Lsy

slot (flare). L =length of slot While th ecarburizing step described embodies the technique of pack-c'arburizing, it will be understood that other systems for carburizing can be employed and that, instead of carburizing, elevated temperature reduction can be carried'o'ut after the steel has been treated by other diffusion transfer techniques, such as cyaniding, nitriding, carlzqctnitriding, chromizing, and the like.

In the described system, the depth of case, the carbon content of the case, the grain size, the rate of carburization, and the carbon gradient have some effect on the increase in endurance limit and wear resistance, as well as on some of the other properties of the steel. The carburizing temperature of 1600" F. was arbitrarily selected as representative of a number of temperatures which might have been employed above the transformationrange of the steel. The temperature of 1600 perrnits good diffusion of the steel and results in a smooth "gradient of carbon from the case to the core.

It will be apparent from the results that the new combination of a diffusion process coupled with working at elevated temperature provides for improvements in the properties of the steel over conventional carburized steels or steels which are cold drawn or elevated temperature drawn as defined in the aforementioned copending appli- Table I Difiusion process of carburizing quenched 20 indicated temperature Drawn to take a 9.8% reduction Air-cooled after drawing Garb. Quench Time in i Tensile Yieid' Elong., Red. or Hardness, DPN t ice. t fee. qzfzench Temp (I) draw, Ilgil, strengith, strengifh, 1.4", area, emp. emu. ce. s. .s. .s. rper- F. F. sec. p p 1 i nt cent 8 MR 0 I Data at approximate peak 01 ETD curve=630 F.

Table II Dlfiusion process of carburizing quenched to indicated temperature Drawn to take 9.8% reduf-tion Quenched in oil after drawing Garb. Quench Time in Tensile Yield Elong., Red. of Hardness, DPN

fee. fee. quench Temp. of draw, Pull, strength, strength, 1.4", area, temp temp., rce., F. lbs. 9.51. p.s.1. perper- Sec cent cent 8 MR 0 H. R 108, 000 70, 500 23. 0 46. 1 213 220 234 OD 9, 160 122. 250 112, 500 11. 0 37. 0 241 258 258 ETD 7, 442 137.000 137.000 8.0 35.8 291 285 276 As Q. (W)- 290. 000 236. 500 6.0 20.0 677 492' 458 As Q. (0) 246. 000 179. 000 0. 7 4.0 565 450 436 1 Data at approximate peai: of ETD eurve=630 F.

Table III Diffusion process of carburizing Air-cooled to room temperature Drawn to take a 9.8% reduction Air-cooled after drawing.

Tensile Yield Elong.. Red. 01 Hardness, DPN Temp. of draw, F. Pull, lbs. strength, strength, 1.4", area,

p.s.i. p.s.i. percent percent 1 Air-cooled carburized steel which is subsequently drawn at temperatures indicated on [allowing lines.

Table VIII Dltiuslon process of earbnrlzlng AlltiOOlBd to room temperature Drawn to take a 12.5% reduction Quenched in oil alter drawing Tensile Yield Elong., Red. Hardness. DPN Temp. of draw, F. Pull, lbs. strength, strength, 1.4", area,

p.s.i. p.s.i. percent percent cations. Favorable characteristics, such as improved endurance limit, wear resistance, surface finish, and corrosion resistance are secured at the high strength levels characteristic of elevated temperature reduction, as described in the aforementioned copending applications. Comparable improvements were made available in the steels which were air-cooled after carburizing and then re-heated to the temperature desired for the elevatedtemperature-reduction step.

It will be understood that the term bar stock, as used in the specification, may include bars, rods, wire, tubing, and the like metal products. It will be further understood that modifications may be made with respect to the carburizing steps, the time and temperature of quench, the temperature of the steel during the elevatedtemperature-reduction step, and the after treatment, as Well as the methods for heating, re-heating, and quenching, without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. The metallurgical process for treating steel of the non-austenitic type having a pearlitic structure in a matrix of free ferrite comprising the combination of steps of heating the steel to a temperature above its transformation range while in surface contact with the material for diffusion transfer onto the surface of the steel, cooling the steel and subsequently taking a reduction in crosssectional area by working the steel while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

2. The metallurgical process for treating steel of the non-austenitic type having a pearlitic structure in a matrix of free ferrite comprising the combination of steps of carburizing the steel and then advancing the carburized steel through a die to take a reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

3. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of pre cipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of heating the steel to a temperature above its transformation range while in contact with a material for diffusion transfer into the steel, cooling the steel and subsequently taking a reduction in cross-sectional area by working the steel while the steel is at a temperature within the range of 450 F. to the lower critical temperature for the steel composition.

4. The metallurgical process fortreating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of heating the steel to a temperature above its transformation range while in contact with a material for ditfusion transfer into the steel, quenching the steel to ambient temperature and subsequently taking a reduction in cross-sectional area by working the steel while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

5. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of heating the steel to a temperature above its transformation range while in contact with a material for diffusion transfer into the steel, air-cooling the steel from the temperature of diifusion and subsequently taking a reduction in crosssectional area by working the steel while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

6. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of carburizing the steel and then advancing the steel through a die to take a reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

7. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of carburizing the steel and then advancing the steel through a draw die to take a reduction in cross-sectional area in a drawing operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

8. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of carburizing the steel and then advancing the steel through an extrusion die to take a reduction in cross-sectional area by an extrusion operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

9. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of carburizing the steel and then rolling the steel to efiect a reduction in cross-sectional area in a rolling operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

10. A steel product having new and improved characteristics produced by the method of claim 6.

11. The metallurgical process for treating steel which strain-hardens and which hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition comprising the combination of steps of heating the steel to a temperature above its transformation range while in contact with a material for difiusion transfer into the steel, cooling the steel and subsequently taking a reduction in cross-sectional area by working the steel while the steel is at a temperature Within the range of 200 F. to the lower critical temperature for the steel composition.

12. The method as claimed in claim 11 in which the steel is air-cooled subsequent to the elevated temperature reduction step.

13. The method as claimed in claim 11 in which the steel is quenched following the elevated temperature reduction step.

12 References Cited in the tile of this patent UNITED STATES PATENTS Potter Feb. 20, 1912 Hibbert Nov. 28, 1950 OTHER REFERENCES I evons: The Metallurgy of Deep Drawing and Pressing, 2nd ed., pages 611-612, 1942.

Metals Handbook, 1948 ed., pages 681-685.

Pomp: The Manufacture and Properties of Steel Wire. Copyright 1941, English translation, 1954. Pages 92-93. 

1. THE METALLURGICAL PROCESS FOR TREATING STEEL OF THE NON-AUSTENITIC TYPE HAVING A PEARLITIC STRUCTURE IN A MATRIX OF FREE FERRITE COMPRISING THE COMBINATION OF STEPS OF HEATING THE STEEL TO A TEMPERATURE ABOVE ITS TRANSFORMATION RANGE WHILE IN SURFACE CONTACT WITH THE MATERIAL FOR DIFFUSION TRANSFER ONTO THE SURFACE OF THE STEEL, COOLING THE STEEL AND SUBSEQUENTLY TAKING A REDUCTION IN CROSSSECTIONAL AREA BY WORKING THE STEEL WHILE THE STEEL IS AT A TEMPERATURE WITHIN THE RANGE OF 200* F. TO THE LOWER CRITICAL TEMPERATURE FOR THE STEEL COMPOSITION. 